Semiconducting behavior of Ag2Te thin films and the dependence of band gap on thickness
TL;DR: In this paper, the electrical resistance of Ag2Te films has been measured as a function of temperature during heating, which was carried out immediately after the film formation, and the observed exponential decrease of resistance with temperature up to the transition point points to the semiconducting nature of the low temperature polymorph of ag2Te.
Abstract: Thin films of Ag2Te of various thicknesses in the range 500–1500 A have been prepared by thermal evaporation of the compound under vacuum on clean glass substrates held at room temperature. The electrical resistance of the films has been measured as a function of temperature during heating, which was carried out immediately after the film formation. The observed exponential decrease of resistance with temperature up to the transition point points to the semiconducting nature of the low temperature polymorph of Ag2Te. The band gap of the low temperature phase is calculated for various thicknesses of the films and it is found that the band gap is a function of film thickness, increasing with decreasing thickness. The increase in the band gap, which was found to be inversely proportional to the square of the film thickness, is attributed to quantization of electron momentum component normal to film plane.
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TL;DR: In this article, a solution-phase synthesis of silver telluride and bismuth-telluride nanowire heterostructure using tellurium (Te) nanowires as sacrificial template and site-selective conversion strategy is reported.
Abstract: In this paper, we report a solution-phase synthesis of silver telluride and bismuth telluride nanowire heterostructure using tellurium (Te) nanowire as sacrificial template and site-selective conversion strategy. High-resolution transmission electron microscope studies confirm sharp interface with possible epitaxial growth between silver telluride and bismuth telluride regions. Through tuning the precursor amount (bismuth and silver precursors) during the reaction, the composition between silver telluride and bismuth telluride can be adjusted. Moreover, the mass produced powder of nanowire heterostructure is consolidated into nanocomposite pellets, and thermoelectric properties of the nanocomposite pellets are investigated between 300 and 400 K. Results show that our materials are p-type with reduced lattice thermal conductivity and a ZT of ∼0.41 at 400 K, which is the best reported value for p-type silver telluride.
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TL;DR: In this article, Das and Karunakaran showed that the phase transition temperature of thin films is a function of thickness increasing with a decrease in the thickness of the thin films.
Abstract: Thin films of silver selenide (Ag2Se) between thicknesses of about 700 and 2200 A have been prepared on glass substrates at room temperature in a vacuum of 5×10−5 Torr. After vacuum annealing the films (at about 373 K for 3 h) electrical resistivity measurements on these films have been carried out in vacuum. From the increase in the rate of decrease of resistance with temperature, the phase transition temperatures (orthorhombic to body‐centered cubic) of the different films have been located. It is found that the phase transition temperature of the thin films is a function of thickness increasing with a decrease in the thickness. This observation has been explained by a recently developed theory [V. Damodara Das and D. Karunakaran, J. Phys. Chem Solids 46, 551 (1985)] of phase transitions in thin films modified further. Also, an order‐of‐magnitude value of the difference in the function of specific surface and interfacial energies of the two phases has also been determined using the theory.
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TL;DR: In this article, it was observed from the studies of electrical resistivity of films of SnSe of different thicknesses and substrate temperatures that resistivity decreases with increase in thickness and substrate temperature.
Abstract: It is observed from the studies of electrical resistivity of films of SnSe of different thicknesses and substrate temperatures that resistivity decreases with increase in thickness and substrate temperature. The study of variation in band gap with thickness shows a linear relationship between band gap and 1/t2. Finally, band gap – substrate temperature studies show that the band gap increases with substrate temperature.
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TL;DR: In this article, the dependence of substrate conditions and the effective thickness of MoO3 films on the morphology of nanostructures and their structural aspects were reported, and the electron microscopy measurements show that the length and the aspect ratio of Nanobibbons increased by 260% without any significant change in the width for a change in effective thickness from 5´nm to 30´nm.
25 citations
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TL;DR: In this paper, the structural transformation in thin films of binary alloys of 1st and 6th group elements, prepared by vacuum evaporation, has been studied by electron diffraction.
Abstract: Structural transformation in thin films of binary alloys of 1st and 6th group elements, prepared by vacuum evaporation, has been studied by the author in detail by electron diffraction. The alloys chosen for the investigation were Ag2Te, Ag2Se, Ag2S, Cu2Te and Cu2Se. The study of thin films of these alloys revealed that there occurs a transformation in structure and on heating, the temperatures of transformation are higher compared to the alloys in the bulk state. It has also been observed that the temperatures of structural transformation during heating and cooling are different. No quantitative explanation of the result has been given and it is expected that detailed discussion of the results will be published later.
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TL;DR: In this paper, the authors investigated the energy gap of the modification of silver selenide stable below 133 ε-C and found that the gap increases as the temperature decreases, indicating that the band-to-band transitions observed are direct.
Abstract: The energy gap of the modification of silver selenide stable below 133\ifmmode^\circ\else\textdegree\fi{}C has been investigated. Studies of the optical-absorption spectrum at 80\ifmmode^\circ\else\textdegree\fi{}K and at 5\ifmmode^\circ\else\textdegree\fi{}K and of the temperature variation of the Hall coefficient and electrical conductivity have been made on polycrystalline samples. Analysis of the data on the electrical properties in the intrinsic region suggests the existence of two phases, not distinguishable by x-ray diffraction, with energy gaps ${E}_{0}$ at 0\ifmmode^\circ\else\textdegree\fi{}K of 0.07\ifmmode\pm\else\textpm\fi{}0.01 eV and 0.18\ifmmode\pm\else\textpm\fi{}0.01 eV, respectively. The latter value agrees with that proposed for the ${\ensuremath{\beta}}_{2}$ phase of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ag}}_{2}\mathrm{Se}$ reported by Baer. The optical-absorption spectra suggest that the band-to-band transitions observed are direct. The extrapolated threshold of absorption is at 0.13 eV at 5\ifmmode^\circ\else\textdegree\fi{}K and correlates best, because of the probable Burstein shift of the absorption edge, with the proposed value of ${E}_{0}=0.07$ eV. It is also concluded that the energy gap increases as the temperature decreases.
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TL;DR: An electron diffraction study has been made on vacuum-deposited films of Ag2Se and Ag2Te on different substrates at various temperatures and the transition of α to β phase occurred reversibly at about 136 °C as mentioned in this paper.
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TL;DR: In this paper, the T-Abhängigkeit der elektrischen Leitfähigkeit and galvanomagnetischen Effekte an stöchiometrischen n-leitenden Proben der Tieftemperatur- or β-phase lassen auf eine kovalent-metallische Bindung schließen.
Abstract: Das Silbertellurid Ag2Te erfährt bei 150° C eine Phasenumwandlung, die mit einer starken Änderung der elektrischen Eigenschaften verknüpft ist. Ausgedehnte Untersuchungen über die T-Abhängigkeit der elektrischen Leitfähigkeit und der galvanomagnetischen Effekte an stöchiometrischen n-leitenden Proben der Tieftemperatur- oder β-Phase lassen auf eine kovalent-metallische Bindung schließen. Fremdatome aus der 4. Spalte (Ge, Sn) und der 5. Spalte (Sb) des Perodischen Systemes haben großen Einfluß auf die Ladungsträger-Konzentration und die Beweglichkeit μ (z. B. in Ag2Te + 1,87 Gew.-% Ag2Ge: µ = 6400 cm2/Vsec, gegenüber 5000 cm2/Vsec in reinem Ag2Te bei 295° K). Bei der Hochtemperatur- oder α-Phase folgt aus der Temperatur- und Stromdichte-Abhängigkeit der Hall-Spannung, daß sich außer Elektronen nach Maßgabe des Te-Überschusses auch Ag-Ionen am Leitungsmechanismus beteiligen. Die ausgeprägte Aktivierungsenergie von 0,98 eV wird im Zusammenhang mit Temperversuchen an Ag2Te in Te-Dampf bei hohen Temperaturen gedeutet. Die Meßergebnisse für die exponentielle T-Abhängigkeit der Beweglichkeit werden an Hand der neuen Theorie von Howarth und Sondheimer für polare Halbleiter diskutiert. Das führt auf die kleine scheinbare Masse m* = 0,4 m. Die Bestimmung des Bandabstandes bei der β-Phase ist wegen der bei 150° C eintretenden Phasenumwandlung aus Leitfähigkeits-Temperaturkurven nicht möglich. Man ist daher auf optische Methoden angewiesen. So wird die Breite der verbotenen Zone aus der Absorptionskante des β-Ag2Te an auf Quarz und Steinsalz aufgedampften dünnen Schichten ermittelt. Dabei werden Ergebnisse über Reflexions- und Transmissionsmessungen von 1 bis 13 µ mitgeteilt. Die sorgfältige Überprüfung der Struktur, des Fehlordnungsgrades und der Stöchiometrie der dünnen Schichten durch röntgenographische, elektrische und optische Messungen sowie der Einfluß einer Wärmebehandlung auf die Absorptionskante werden im Zusammenhang mit den Meßkurven diskutiert. Die optischen Konstanten sind für die zehn besten der über fünfzig untersuchten Schichten aus den exakten transzendenten Gleichungen der Maxwellschen Theorie berechnet worden. Als Grenzwellenlänge der Absorption ergibt sich ein Wert von 1,85 µ entsprechend 0,67 eV. Der Brechungsindex noo beträgt 3,4.
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